Roll-up doors are used in a variety of industrial applications, typically for the purpose of separating areas within a building, or closing off building entries from the outside. A typical roll-up door comprises a fabric curtain which is wound about a roller journalled for rotation above the doorway with which the roll-up door is associated. To close the door, the roller is rotated such that the curtain pays off of the roller to enclose the doorway. Of course, the door is opened by reversing the direction of the roller and rolling the fabric curtain onto the roller. Such roller doors are typically either powered opened and closed, or are powered open and allowed to fall closed by gravity.
When the roll-up door is placed over an exterior doorway of a building, provision must be made to prevent the fabric curtain from billowing due to wind being applied from the outside. Similarly, when the roll-up door is in place between different sections of a warehouse, there may be pressure differentials between these two sections, which may also cause billowing of the roll-up door if the door does not have provision to prevent this from happening. Such billowing is problematic because it affects door function and may allow leakage past the door. To correct for this problem, roll-up doors typically include bottom member in the form of a rigid or semi-rigid bottom bar to provide so-called "wind retention" or prevention of undesirable billowing. The bottom bar typically extends across the leading width of the door, and also includes extensions which extend past either side of the door. These extensions are typically received within side frames disposed on either side of the door and run vertically along the side of the doorway. As the door moves between its open and closed positions, the bottom bar and its extensions or "side frame inserts" move within a generally vertical plane since they are guided within the generally vertical side frames which may include a guide track for that purpose. With the leading edge of the door thus restrained within a vertical plane, movement of the fabric curtain of the door out of that vertical plane is largely avoided. However, the bottom bar only ensures that the leading edge of the door stays in the vertical plane, and strong gusts of wind or large pressure differentials between sections of a building may still allow the remainder of the curtain to billow either during the curtain's travel, or when it is fully closed.
To prevent this undesirable movement of the door, many prior art doors use a tensioning means to place a vertically disposed tension on the door to prevent it from billowing out of the vertical plane. One example of such a tensioning means is a heavy bottom bar. The weight of the heavy bottom bar may provide sufficient vertical tension to prevent undesirable billowing particularly in a gravity-fall type door. Alternatively, external means may be used to provide the necessary tension. For example, belting is often used for this purpose. Typically, one end of the belting is attached to a roller pulley, and is wound and unwound from the roller in the opposite sense from the curtain. The belt is then passed through a pulley mounted near the bottom of the side frame. The other end of the belt is then attached to the side from insert of the bottom bar. As the belt is wound and unwound from the roller in an opposite sense to the curtain, it exerts a downward pulling force on the bottom bar and the side frame inserts thus placing the necessary vertical tension on the door. This pulling force may be enhanced by a torsion spring disposed in the roller and engaging the roller pulley as in U.S. Pat. No. 4,887,660 which is assigned to the Assignee of the present invention. Forces may also be applied directly to the belt as in U.S. Pat. No. 4,997,022. Other particular arrangements for the belting besides those previously described are also used to achieve the same purpose.
A further exemplary means for exerting the necessary vertical tension on the door, at least in the closed position, is a system wherein the side frame inserts of the bottom bar are latched in position when the door is in the closed position. In the case of the powered roll-up door, the motor is then reversed to exert the necessary vertical tension of the door to hold it taut.
While the variety of methods just described for providing wind retention are generally effective for that purpose they are not without their own disadvantages. For example, obstacles in the path of travel of the bottom bar may be problematic. If an obstacle is in place in this position, and the door continues its downward movement, damage to either the door or the object could occur. Further, if the obstacle should be warehouse or other personnel, either damage to the door or injury to the personnel could result. To avoid this problem, doors employing bottom bars typically also include some type of sensing mechanism for determining when an obstacle has been encountered. These sensors are coupled to the motor which drives the roller, and cause the door to be reversed upon encountering an obstacle.
Since this type of door is often used in a warehouse environment, where forklifts are employed, roll-up doors are also subject to being struck by such forklifts, thus putting an unwanted horizontal or other impact force on the door. Of course, other sources of such forces besides forklifts may also be encountered. Typically, such impacts on the door occur while the door is in the middle of its range of travel. A simple rigid bottom bar with side frame members extending beyond the edge of the door and into side frames, will be subject to damage upon impact. That is, it could either be bent or broken, in either event probably requiring replacement. Alternatively, if the bottom bar is rigid enough, damage to the object striking it may occur. To avoid this problem, many bottom bars include a breakaway mechanism, that allow the bottom bar attached to the bottom of the door to be separated from the side frame inserts upon application of a horizontal force above a certain magnitude. Examples of such breakaway mechanisms are found in the following U.S. Pat. Nos.: 5,271,448 and 5,025,847. Since the side frame inserts typically become separated from the bottom bar in these breakaway mechanisms, they must be reassembled after a breakaway condition occurs. In the case where the side frame inserts are attached to belts the other end of which is attached to the roller, such reattachment can be problematic since the elasticity of the belts must be overcome to re-place the side frame inserts adjacent to the bottom bar. Further, regardless of whether the side frame inserts are attached to belting, the breakaway condition may be dangerous since the side frame inserts can fall by gravity and potentially injure personnel below. Thus while the presence of a breakaway mechanism can avoid the need to replace the bottom bar upon each impact, employment of such a breakaway mechanism is not without its own disadvantages. Further, such breakaway mechanisms typically allow for breakaway only upon application of a horizontal force perpendicular to the plane of the door. Provision is typically not made for breakaway due to either a vertical force or forces having both horizontal and vertical components, and such doors still require a sensor or other means, described above, for preventing impact with obstacles in the doorway.